The present invention relates to data compression and watermarking.
In computing, a watermark is a bit-sequence that relates or is associated with other digital content. Unlike printed watermarks consisting of specially printed images or embossed papers, digital watermarks may also correspond to general number sequences, checksums or any other type of digitally transmitted data. Resilient digital watermarks can be used to identify the owner of certain content. These watermarks continue to identify a particular owner despite attempts by a third party to remove or alter the content without authorization. In contrast, fragile watermarks become unreadable or corrupt when digital content and the underlying watermark are altered. These fragile watermarks ensure the authenticity of certain documents and prevent data tampering from going unnoticed. No doubt, many other uses of digital watermarks exist and will be created in the future.
In most cases, the watermark is transmitted along with an input stream of digital data. If the input stream is not compressed, the watermark can be added to the digital data input stream increasing the overall amount of data being transmitted. Often however, it is desirable to compress the digital data stream thereby increasing the effective network bandwidth and decreasing storage requirements. One solution is to compress the input stream using a compression routine and then add the watermark bit sequence.
Adding a watermark to compressed digital data is not particularly difficult using lossy compression techniques. These lossy compression techniques selectively eliminate certain bits and replace them with the watermark values, or otherwise alter the encoded data to introduce the watermark information. Data loss in the original data stream is not only caused by the lossy compression/decompression operation but exacerbated by the addition of the watermark. In some cases, the loss introduced by the addition of a watermark is acceptable.
Unfortunately, adding a watermark and loss to an input data stream compressed losslessly defeats the purpose of lossless compression. Popular lossless compression methods such as the Lempel-Ziv (LZ) schemes do not provide space for watermark insertion. Resolving this problem is even more challenging if an inserted watermark cannot reduce compression performance and must also preserve the size of the compressed stream. Even if the watermarks could be introduced without introducing loss or compression performance deterioration, it would be difficult to do so and ensure that the enhanced decompressors be backward compatible with existing coders and compressed streams. Lack of backward compatibility stands as an impediment to enhancing lossless compression operations to accept watermarks.
Like reference numbers and designations in the various drawings indicate like elements.